The present invention relates to a method for polymerizing alpha-olefins, a catalyst for such a polymerization method and a method for producing such a catalyst. In particular, the present invention relates to a catalyst, and a method for preparation thereof, which produces linear low density polyethylene (LLDPE) having a narrow molecular weight distribution, as evidenced by relatively low values of melt flow ratio, (MFR), and low hexane extractables, suitable for film and injection molding applications. The invention is also directed to a highly productive polymerization process carried out with the catalyst of the invention. 2. Description of the Prior Art
Linear low density polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as homopolymers of polyethylene. Certain of these properties are described in Anderson et al, U.S. Pat. No. 4,076,698.
Karol et al, U.S. Pat. No. 4,302,566, describe a process for producing linear low density polyethylene polymers in a gas phase, fluid bed reactor.
Graff, U.S. Pat. No. 4,173,547, Stevens et al, U.S. Pat. No. 3,787,384, Strobel et al, U.S. Pat. No. 4,148,754, and Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, each describe various polymerization processes suitable for producing forms of polyethylene other than linear low density polyethylene, per se.
Graff, U.S. Pat. No. 4,173,547, describes a supported catalyst obtained by treating a support with both an organoaluminum compound and an organomagnesium compound followed by contacting this treated support with a tetravalent titanium compound.
Stevens et al, U.S. Pat. No. 3,787,384, and Strobel et al, U.S. Pat. No. 4,148,754, describe a catalyst prepared by first reacting a support (e.g., silica containing reactive hydroxyl groups) with an organomagnesium compound (e.g., a Grignard reagent) and then combining this reacted support with a tetravalent titanium compound. According to the teachings of both of these patents, no unreacted organomagnesium compound is present when the reacted support is contacted with the tetravalent titanium compound.
Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, describe a catalyst which is the reaction product of an organomagnesium compound (e.g., an alkylmagnesium halide) with a tetravalent titanium compound. The reaction of the organomagnesium compound with the tetravalent titanium compound takes place in the absence of a support material.
A vanadium-containing catalyst, used in conjunction with triisobutylaluminum as a co-catalyst, is disclosed by W.L. Carrick et al in Journal of American Chemical Society, Volume 82, page 1502 (1960) and Volume 83, page 2654 (1961).
Nowlin et al, U.S. Pat. No. 4,481,301, disclose a supported alpah-olefin polymerization catalyst composition prepared by reacting a support containing OH groups with a stoichiometric excess of an organomagnesium composition, with respect to the OH groups content, and then reacting the product with a tetravalent titanium compound.
Dombro, U.S. Pat. Nos. 4,378,304 and 4,458,058, disclose an olefin polymerization catalyst composition synthesized by sequentially reacting: (1) a porous support with a Group IIA organometallic compound, e.g., a dialkylmagnesium; (2) the product of (1) with water or a hydrocarbyl alcohol, e.g., methanol; (3) the product of (2) with a transition metal compound or compounds. The product of the synthesis reaction is activated with a co-catalyst which is a Group IA, IIa, IIIA and/or IIB organometallic compound, including hydrogen. Suitable co-catalysts are n-butylithium, diethylmagnesium, triisobutylaluminum and diethylaluminum chloride.
Best, U.S. Pat. Nos. 4,558,024, 4,558,025 and 4,579,835, disclose olefin polymerization catalyst compositions prepared by reacting together a porous particulate material, an organic magnesium compound, an oxygen-containing compound, a transition metal compound, e.g., a titanium compound (the U.S. Pat. No. 4,558,024) or a vanadium compound (the U.S. Pat. No. 4,579,835), and a co-catalyst. Some of the catalyst compositions of Best also include an acyl halide (e.g, see the U.S. Pat. No. 4,579,835 and the U.S. Pat No. 4,558,025).
When the LLDPE resins are fabricated into injection-molded products, it is imperative to assure that such products are not susceptible to warping or shrinking. As is known to those skilled in the art, the degree of warping or shrinking can be predicted from the molecular weight distribution of the resins. Resins having relatively narrow molecular weight distribution produce injection-molded products exhibiting a minimum amount of warping or shrinkage. Conversely, resins having relatively broad molecular weight distribution produce injection-molded products more likely to undergo warping or shrinkage. One of the measures of the molecular weight distribution of the resin is melt flow ratio (MFR), the ratio of high melt flow index (HLMI or I.sub.21) to melt index (I.sub.2) for a given resin. The melt flow ratio is believed to be an indication of the molecular weight distribution of the polymer, the higher the MFR value, the broader the molecular weight distribution. Resins having relatively low MFR values, e.g., of about 20 to 50, have relatively narrow molecular weight distribution. Additionally, LLDPE resins having such relatively low MFR values produce films of better strength properties than resins with high MFR values. Many catalyst systems exhibit a tendency to produce resins whose MFR values, although initially low, increase with increased concentration of the catalyst activator, also known as a co-catalyst, such as various aluminum alkyls. Under certain circumstances, e.g., to improve catalyst productivity and/or comonomer incorporation, it is desirable to increase the catalyst activator concentration without substantially affecting the resin MFR.
Another important property of LLDPE resins, manufactured into products coming into contact with articles subject to FDA regulations, e.g., foodstuffs, is hexane extractables which is a measure of the amount of low molecular weight and/or highly branched polymer fractions capable of being extracted from the manufactured products, e.g., plastic food containers, by hexane extraction. The FDA imposed strict regulations on the amounts of allowable hexane extractables in such plastic products.
Thus, Allen et al, European Patent Office (EPO) Application No. 87300536.1, published on Aug. 5, 1987, as publication No. 0231102, disclose an alpha-olefin polymerization catalyst composition activated with trimethylaluminum which produces polymers having relatively low values of MRF and low hexane extractables. However, the productivity of the polymerization process carried out with such a catalyst composition is lower than that of the process carried out with the same catalyst compositions activated with more commonly-used activators, such as triethylaluminum and triisobutylaluminum.
Accordingly, it is important to provide a catalyst composition capable of producing a alpha-olefin polymers having relatively narrow molecular weight distribution (low MFR values) which remains substantially constant with varying amounts of the co-catalyst, and which catalyst composition has high activity.
It is therefore a primary object of the present invention to provide a high activity catalyst for the polymerization of alpha-olefins yielding products of a relatively narrow molecular weight distribution which is maintained substantially constant with varying amounts of the co-catalyst concentration.
It is another object of the present invention to provide a high activity catalyst composition which produces alpha-olefin polymers having relatively low hexane extractables.
It is an additional object of the present invention to provide a catalytic process for polymerizing alpha-olefins which yields linear low density polyethylene of a relatively narrow molecular weight distribution at high productivity rates.